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Large parts of Australia are facing a hotter and drier summer than average, according to the Bureau of Meteorology’s summer outlook.

Drier than average conditions are likely for much of northern Australia. Most of the country has at least an 80% chance of experiencing warmer than average day and night-time temperatures.

The threat of bushfire will remain high, with few signs of the sustained rain needed to reduce fire risk or make a significant dent in the ongoing drought.

Expect extreme heat

Large parts of Western Australia, most of Queensland and the Top End of the Northern Territory are expected to be drier than usual. Further south, the rest of the country shows no strong push towards a wetter or drier than average summer, which is a change for parts of the southeast compared to recent months.

Bureau of Meteorology

Queensland has already seen some extraordinary record-breaking heat in recent days, with summer yet to truly begin. With the summer outlook predicting warmer days and nights, combined with recent dry conditions and our long-term trend of increasing temperatures, some extreme highs are likely this summer.

Bureau of Meteorology

All of this means above-normal bushfire potential in eastern Australia, across New South Wales, Victoria and Queensland. The bushfire outlook, also released today, notes that rain in areas of eastern Australia during spring, while welcome, was not enough to recover from the long-term dry conditions. The current wet conditions across parts of coastal New South Wales will help, but it will not take long once hot and dry conditions return for vegetation to dry out.

What about El Niño?

However, not all the ducks are lined up. While ocean temperatures have already warmed to El Niño levels, to declare a proper “event” there must also be a corresponding response in the atmosphere to reinforce the ocean – this hasn’t happened yet.

That said, climate models expect this event to arrive in the coming months. The outlook has factored in that chance, and the conditions predicted are largely consistent with what we would expect during El Niño. In summer, this includes drier weather in parts of northern Australia, and warmer summer days.

Once an El Niño is in place, weather systems across southern Australia tend to be more mobile. This can mean shorter but more intense heatwaves in Victoria and southern South Australia. However, in New South Wales and Queensland, El Niño is associated with both longer and more intense heat waves.

The exact reason why the states are affected differently is complicated, but relates to the fast-moving cold fronts and troughs that sweep through Victoria and South Australia in the summertime, creating cool changes. These weather systems don’t influence areas further north so when hot air arrives, it takes longer to clear.

The heavy rains seen in parts of eastern Australia in October and November have provided some welcome short-term relief to drought-stricken farmers, but longer-term rainfall relief has not arrived yet. If El Niño arrives, this widespread relief may only be on the cards in autumn.

Once the warm weather arrives, you know mosquitoes won’t be far behind. Spring heatwaves associated with the impending arrival of El Niño to the east coast of Australia may mean we’ll get an early taste of summer, but what about mosquitoes? Does a long, hot summer mean fewer annoying buzzing and biting beasts bothering us whenever we spend time outdoors?

Where do mosquitoes come from?

Mosquitoes are complex animals. Like all insects, they thrive in warm weather, but they need more than just heat, they need water.

Mosquitoes lay their eggs on or around water. Without it, they cannot complete their life cycle. Mosquito “wrigglers” hatch out from eggs and spend a week or so swimming about before emerging and flying off in search of blood. Depending on where the water is, whether it is wetlands, puddles or water-filled containers, different kinds of mosquitoes will be present.

There are hundreds of different mosquitoes in Australia. Some like salty water, some like fresh. Some need pristine conditions while some will tolerate filthy water trapped at the bottom of a septic tank.

Because mosquitoes rely on water, rainfall plays a critical role in determining how many mosquitoes will be buzzing about this summer.

While floods bring mosquitoes, and often outbreaks of mosquito-borne disease, drought will knock out almost all mosquitoes. It is true that the ongoing dry conditions across inland areas of Australia will ensure mosquito populations remain low, but that doesn’t mean mosquitoes will disappear completely.

Water doesn’t just come from rain

While a lack of rain will keep many wetlands dry, that isn’t the case for our coastal wetlands. Some of the worst pest mosquitoes in Australia are found in our mangroves, saltmarshes and sedgelands.

Mosquitoes, like the saltmarsh mosquito, Aedes vigilax, love wetlands regularly flooded by high tides. The eggs of this mosquito, laid in moist wetland mud, survive long periods of dry conditions. Once covered by tides, these hatch, complete development within a week, and emerge in extraordinary numbers to fly kilometres away into nearby communities to bite and spread disease-causing pathogens such as Ross River virus.

Not only have these mosquitoes found a way to survive without rain, they thrive in hot and dry conditions. Without substantial rainfall, the pools and ponds in the wetlands dry completely, killing off any fish or other aquatic predators, ensuring perfect conditions once the next series of tides comes flooding in. The arrival of El Niño may be bad news for lots of wetland wildlife, but it isn’t all bad news for mosquitoes.

The saltmarsh mosquito, Aedes vigilax, is one of the most important pest mosquitoes in coastal regions of Australia and has adapted to thrive in hot and dry conditions.Stephen Doggett (NSW Health Pathology)

Bringing mosquitoes home

Much has been made of the impact of heatwaves on human health. It may also inadvertently increase health risks in metropolitan regions of Australia. A shortage of water increases the need to conserve and store water around the backyard. Unfortunately, that also means creating a home for mosquitoes.

One of the most widespread mosquitoes in the country, a mosquito that has probably bitten almost every Australian, is the backyard mosquito Aedes notoscriptus. This mosquito is found in water-filled containers around the backyard, from drains and roof gutters to rainwater tanks and bird baths. While you’d think hot and dry conditions will impact this mosquito, think about the extra effort we’re taking to store water around the home. If your rainwater tank isn’t properly screened or you’re keeping uncovered bins and buckets around the backyard filled with water, you’ll be providing a home for mosquitoes.

This decrease in numbers in WA could be linked to an El Niño event that originated far away in the Pacific Ocean, we suggest in a paper published today in Global Change Biology. The findings could have implications for future sudden drops in dolphin numbers here and elsewhere.

A Pacific event

The El Niño Southern Oscillation (ENSO) results from an interaction between the atmosphere and the tropical Pacific Ocean. ENSO periodically fluctuates between three phases: La Niña, Neutral and El Niño.

During our study from 2007 to 2013, there were three La Niña events. There was one El Niño event in 2009, with the initial phase in winter being the strongest across Australia.

The blue vertical line shows the decline in dolphin numbers (d) during the 2009 El Niño event.Kate Sprogis, Author provided

Coupled with El Niño, there was a weakening of the Leeuwin Current, the dominant ocean current off WA. There was also a decrease in sea surface temperature and above average rainfall.

ENSO is known to affect the strength of the south-ward flowing Leeuwin Current.

During La Niña, easterly trade winds pile warm water on the western side of the Pacific Ocean. This westerly flow of warm water across the top of Australia through the Indonesian Throughflow results in a stronger Leeuwin Current.

During El Niño, trade winds weaken or reverse and the pool of warm water in the Pacific Ocean gathers on the eastern side of the Pacific Ocean. This results in a weaker Indonesian Throughflow across the top of Australia and a weakening in strength of the Leeuwin Current.

The strength and variability of the Leeuwin Current coupled with ENSO affects species biology and ecology in WA waters. This includes the distribution of fish species, the transport of rock lobster larvae, the seasonal migration of whale sharks and even seabird breeding success.

The question we asked then was whether ENSO could affect dolphin abundance?

What happened during the El Niño?

These El Niño associated conditions may have affected the distribution of dolphin prey, resulting in the movement of dolphins out of the study area in search of adequate prey elsewhere.

A surfacing bottlenose dolphin.Kate Sprogis/MUCRU, Author provided

This is similar to what happens for seabirds in WA. During an El Niño event with a weakened Leeuwin Current, the distribution of prey changes around seabird’s breeding colonies resulting in a lower abundance of important prey species, such as salmon.

In southwestern Australia, the amount of rainfall is strongly connected to sea surface temperature. When the water temperature in the Indian Ocean decreases, the region receives higher rainfall during winter.

High levels of rainfall contribute to terrestrial runoff and alters freshwater inputs into rivers and estuaries. The changes in salinity influences the distribution and abundance of dolphin prey.

This is particularly the case for the river, estuary, inlet and bay around Bunbury. Rapid changes in salinity during the onset of El Niño may have affected the abundance and distribution of fish species.

Of these strandings, in southwest Australia, there was a peak in June that coincided with the onset of the 2009 El Niño.

Specifically, in the Swan River, Perth, there were several dolphin deaths, with some resident dolphins that developed fatal skin lesions that were enhanced by the low-salinity waters.

What does all this mean?

Our study is the first to describe the effects of climate variability on a coastal, resident dolphin population.

A group of bottlenose dolphins.Kate Sprogis/MUCRU, Author provided

We suggest that the decline in dolphin abundance during the El Niño event was temporary. The dolphins may have moved out of the study area due to changes in prey availability and/or potentially unfavourable water quality conditions in certain areas (such as the river and estuary).

Long-term, time-series datasets are required to detect these biological responses to anomalous climate conditions. But few long-term datasets with data collected year-round for cetaceans (whales, dolphins and porpoises) are available because of logistical difficulties and financial costs.

Continued long-term monitoring of dolphin populations is important as climate models provide evidence for the doubling in frequency of extreme El Niño events (from one event every 20 years to one event every ten years) due to global warming.

With a projected global increase in frequency and intensity of extreme weather events (such as floods, cyclones), coastal dolphins may not only have to contend with increasing coastal human-related activities (vessel disturbance, entanglement in fishing gear, and coastal development), but also have to adapt to large-scale climatic changes.

You’ve probably heard about El Niño, the climate system that brings dry and often hotter weather to Australia over summer.

You might also know that climate change is likely to intensify drought conditions, which is one of the reasons climate scientists keep talking about the desperate need to reduce greenhouse gas emissions, and the damaging consequences if we don’t.

El Niño is driven by changes in the Pacific Ocean, and shifts around with its opposite, La Niña, every 2-7 years, in a cycle known as the El Niño Southern Oscillation or ENSO.

But that’s only part of the story. There’s another important piece of nature’s puzzle in the Pacific Ocean that isn’t often discussed.

It’s called the Interdecadal Pacific Oscillation, or IPO, a name coined by a study which examined how Australia’s rainfall, temperature, river flow and crop yields changed over decades.

Since El Niño means “the boy” in Spanish, and La Niña “the girl”, we could call the warm phase of the IPO “El Tío” (the uncle) and the negative phase “La Tía” (the auntie).

These erratic relatives are hard to predict. El Tío and La Tía phases have been compared to a stumbling drunk. And honestly, can anyone predict what a drunk uncle will say at a family gathering?

What is El Tío?

Like ENSO, the IPO is related to the movement of warm water around the Pacific Ocean. Begrudgingly, it shifts its enormous backside around the great Pacific bathtub every 10-30 years, much longer than the 2-7 years of ENSO.

The IPO’s pattern is similar to ENSO, which has led climate scientists to think that the two are strongly linked. But the IPO operates on much longer timescales.

We don’t yet have conclusive knowledge of whether the IPO is a specific climate mechanism, and there is a strong school of thought which proposes that it is a combination of several different mechanisms in the ocean and the atmosphere.

Despite these mysteries, we know that the IPO had an influence on the global warming “hiatus” – the apparent slowdown in global temperature increases over the early 2000s.

Global temperatures are on the up, but the IPO affects the rate of warming.Author provided, data from NOAA, adapted from England et al. (2014) Nat. Clim. Change

Temperamental relatives

When it comes to global temperatures we know that our greenhouse gas emissions since the industrial revolution are the primary driver of the strong warming of the planet. But how do El Tío and La Tía affect our weather and climate from year to year and decade to decade?

Superimposed on top of the familiar long-term rise in global temperatures are some natural bumps in the road. When you’re hiking up a massive mountain, there are a few dips and hills along the way.

Several recent studies have shown that the IPO phases, El Tío and La Tía, have a temporary warming and cooling influence on the planet.

In the negative phase of the IPO (La Tía) the surface temperatures of the Pacific Ocean are cooler than usual near the equator and warmer than usual away from the equator.

Since about the year 2000, some of the excess heat trapped by greenhouse gases has been getting buried in the deep Pacific Ocean, leading to a slowdown in global warming over about the last 15 years. It appears as though we have a kind auntie, La Tía perhaps, who has been cushioning the blow of global warming. For the time being, anyway.

The flip side of our kind auntie is our bad-tempered uncle, El Tío. He is partly responsible for periods of accelerated warming, like the period from the late 1970s to the late 1990s.

The IPO has been in its “kind auntie” phase for well over a decade now. But the IPO could be about to flip over to El Tío. If that happens, it is not good news for global temperatures – they will accelerate upwards.

Models getting better

One of the challenges to climate science is to understand how the next decade, and the next couple of decades, will unravel. The people who look after our water and our environment want to know things like how fast our planet will warm in the next 10 years, and whether we will have major droughts and floods.

To do this we can use computer models of Earth’s climate. In our recently published paper in Environmental Research Letters, we evaluated how well a large number of models from around the world simulate the IPO. We found that the models do surprisingly well on some points, but don’t quite simulate the same degree of slow movement (the stubborn behaviour) of El Tío and La Tía that we observe in the real world.

But some climate models are better at simulating El Tío and La Tía. This is useful because it points the way to better models that could be used to understand the next few decades of El Tío, La Tía and climate change.

Figures from the Bureau of Meteorology indicate Australia has experienced its hottest autumn on record.Bureau of Meteorology

The Bureau of Meteorology has reported that for average temperatures across Australia, this has been the hottest March-May period ever recorded – beating the previous record, set in 2005, by more than 0.2℃.

Within this period, March was also the hottest on record, while April and May were each the second-warmest in a series extending back to 1910.

Temperatures were well above average across much of the country, especially in the east.Bureau of Meteorology

Why so hot?

El Niño events tend to cause warmer weather across the east and north of Australia and the major El Niño of 2015-16 undoubtedly contributed to the extreme temperatures experienced across these areas.

However, climate change also played a significant role in our warmest autumn. Previous work, led by ANU climatologist Sophie Lewis, indicates that the human influence on the climate has made a record-breakingly hot autumn roughly 20 times more likely.

In other words, without climate change we would be much less likely to experience autumns as warm as this one has been in Australia.

The extreme heat over Australia this autumn and the associated damage to the reef are also having an effect on the election campaign. As public concern over the future of the reef grows, the parties are being asked to defend their climate change policies.

Both major parties have made election commitments to the reef, with the Coalition announcing an extra A$6 million to tackle crown-of-thorns starfish (adding to a further A$171 million committed under the 2016 budget), and Labor an extra A$377 million over five years (A$500 million in total). While both Labor and the Coalition aim to improve water quality in the reef through their policies, the coral bleaching and death this year is linked with warm seas.

Whether we’ll be able to save parts of the reef largely depends on whether we reduce our greenhouse gas emissions and manage to prevent the rising trend in temperatures from continuing.

As the summer ends, heat is dominating the meteorological landscape, with the warmest month ever recorded and the drought continuing unabated in California. At the same time, it is clear that an El Niño is building that is expected to culminate in the fall and last until the winter, with the possibility of it becoming a “mega” El Niño.

The hope in California is that the large amounts of precipitation usually associated with extreme El Niño events would lessen the impacts of the state’s multi-year drought by partly refilling reservoirs and groundwater, even as scientists caution that this might not happen to the degree needed to alter the present situation.

What drives the El Niño weather pattern and what do scientists know about El Niño under man-made greenhouse warming?

A tropical Pacific phenomenon with global influence

To be clear, El Niño is a tropical Pacific phenomenon, even though it represents the strongest year-to-year meteorological fluctuation on the planet and disrupts the circulation of the global atmosphere. When sea surface temperature changes – or anomalies – in the eastern equatorial Pacific exceed a certain threshold, it becomes an El Niño.

What are the mechanisms behind El Niño? In normal conditions in the tropical Pacific, the trade winds blow from east to west, driving ocean currents westwards underneath. These currents transport warm water that is heated by low-latitude solar radiation and eventually piles up in the western Pacific. As a result, heat accumulates in the upper ocean.

Under normal conditions, winds help carry warm water from east to west.Michael McPhaden/NOAA

The warm water evaporates from the ocean surface, and the light, warm and humid air rises, leading to deep convection in the form of towering cumulonimbus clouds and heavy precipitation. As this air ascends, it reaches upper levels of the troposphere and returns eastwards to eventually sink over the cooler water of the eastern Pacific. This east-west (zonal) circulation is called the Walker Circulation.

What happens to the atmosphere and the ocean during El Niño?

This circulation gets disrupted every few years by El Niño or enhanced by La Niña, the opposite effect. This periodic, naturally occurring phenomenon is called the El-Niño Southern Oscillation (ENSO).

During the typical El Niño, the warm phase of that oscillation, the trade winds weaken, and episodic westerly wind bursts in the western equatorial Pacific generate internal waves into the ocean. These waves trigger the transport of the warm water from the west to the east of the basin.

This induces a reduction of the upwelling (upward motion) of cold water in the east, at the equator and along the coast. It also creates warm sea surface temperature anomalies along the equator from the international dateline in the Pacific to the coast of South America.

As the central part of the Pacific warms up during El Niño, the atmospheric convection that normally occurs over the western warm pool migrates to the central Pacific. That transfer of heat from the ocean to the atmosphere gives rise to extraordinary rainfall in the normally dry eastern equatorial Pacific. Warm air then flows from the west, feeding this convection and further weakening the east-west-flowing trade winds. This leads to further warming as this feedback loop amplifies the phenomenon and ensures that deep atmospheric convection and rainfall patterns are maintained in the central equatorial Pacific. El Niño eventually ends when changes in the ocean cause negative feedbacks that reverse the dynamics that create the El Niño effects.

How can El Niño affect weather in United States and rainfall in California?

In association with El Niño, the heat redistribution in the ocean creates a major reorganization of atmospheric convection, severely disrupting global weather patterns from Australia to India and from South Africa to Brazil.

What explains the specific effect on the US and California, however, is a particular type of connection – called extratropical teleconnections – between the heating generated by El Niño and North America. This heating excites wave trains, or groups of similar-sized atmospheric waves, that propagate northward, connecting the central equatorial Pacific to North America. This shifts the subtropical jet stream northward and induces a series of storms over California and the southern US, in general. The increased precipitation that ensues seems to only occur during a strong El Niño.

While El Niños have a rather “typical” signature in the tropics, their impacts over North America vary because other influences act in temperate climates. Nevertheless, most El Niño winters are mild over western Canada and parts of the northern central United States, and wet with anomalous precipitation over the southern United States from Texas to Florida.

How might El Niño evolve under man-made greenhouse warming?

Scientists are now studying the diversity in El Niño behavior – strong and weak ones, changes in duration, and the different regions for the maximum SST anomalies. Are these changes to El Niño related to global warming? It is too early to say.

Climate models do suggest that the mean conditions in the Pacific will evolve toward a warmer state. That means sea surface temperatures are likely to rise and the trade wind to weaken, which could lead to a more permanent El Niño state and/or more intense El Niño events.

Some climate model projections, together with reconstructions of past El Niños, provide empirical support for more extreme El Niño events under greenhouse warming. They also point toward an eastward shift of the center point where heat from the ocean transfers to the air. This would mean an eastward shift of extratropical rainfall teleconnections, the phenomenon responsible for weather changes in North America, including more rain in the West.

But models diverge in their predictions of whether and how the teleconnections’ intensity will change. So there is no simple answer to how precipitation will change in California in association with changes of El Niño related to greenhouse warming.

A complex phenomenon with many tricks for scientists

Will the sensitivity of the atmosphere to the primary mechanism at the heart of El Niño – that is, feedback between the higher sea temperatures and slowing trade winds, leading to atmospheric convection over the central Pacific – continue in the future?

It was not maintained during 2014, when otherwise favorable conditions for a big El Niño were present. In that case, persistent deep convection did not occur in the central Pacific, and the usual strong interaction between the atmosphere and the ocean there failed to play its normal role in anchoring the convection and heat transfer.

These results show us that we still have much to learn. This is true despite the dramatic scientific progress that has been accomplished over the last few decades regarding El Niño and ENSO cycles, including new theories, sophisticated seasonal forecasting models and extensive observation systems.

Our ability to predict El Niño and the potential connections between increasing greenhouse gases and El Niño is still limited by the complexity of the ENSO dynamics, as exemplified by the failed prediction of a 2014 El Niño. In the meantime, we can look forward to a winter when El Niño, perhaps even a mega El Niño, will dominate the weather discussion.